It is known to form a gas nitrided layer on a piston ring as a base for a metallic nitride face coating. The face coatings complement the hard gas nitrided layer by providing advantages not possessed by the gas nitrided layer, such as strong scuff resistance and extremely high hardness.
Yet, the desirable advantages offered by metallic nitride face coatings result in unwanted side effects. The coatings are particularly prone to chipping and may not achieve suitable coating thickness if they lack sufficiently strong bonds with an underlying substrate. Even if a suitable substrate is provided, however, there is often insufficient surface area between the coating and the substrate to promote the necessary bonds.
The lack of porosity and very high strength results in increased friction between the face coating and a mating mechanical element such as a cylinder wall. Metallic nitride face coatings lack the inherent ability of oil retention. Therefore, they cannot provide a critical source of lubrication during periods when boundary layer lubrication is required and to enhance scuff resistance during piston ring break in.